There is a boisterous boardwalk at Seaside Heights in New Jersey. Along with the pizza shops and miniature golf, there are many shops selling Jersey Shore T-shirts.

"All of them are offering free henna tattoos," said Sandy Pallas, of Bethel.

In August, Pallas and her three children were at Seaside Heights. The kids wanted henna tattoos on their shoulders. Because henna is a natural substance and the tattoos fade, Pallas said, she gave them the OK.

Only it wasn't just henna in the tattoo. There was a darker, chemical dye -- paraphenylenediamine, or PPD -- mixed in.

Within two days, Pallas said, the New York Giants tattoo on her 10-year-old son Dean's shoulder was just one big blister. Her daughter, Kate, 7, got a dolphin over a moon on her shoulder. It also became red and blistery.

When she took then to see Dr. Robert Golenbock, a Danbury pediatrician, he was taken aback.

"They were really pretty bad inflammatory reactions," Golenbock said.

And, he admitted Thursday, he had never heard of black henna or the problems it can cause until he saw the Pallas children.

Golenbock found a website -- www.hennapage.com -- with a section dedicated to the dangers of black henna. He's now circulated it to all area pediatricians, letting them know about the problem.

"If a doctor were to do this, it would be the equivalent of malpractice," Golenbock said of the misuse of the dye.

Danbury dermatologist Dr. William Notaro said patients came to him with black henna problems in the past.

"I haven't seen any cases lately," Notaro said. "But I have seen them."

Henna -- Lawsonia inermis -- is a flowering shrub, native to Africa, Asia and parts of Australia. Humans have used it to dye their hair, their skin, their fingernails and their clothing for millennia.

For body art, powdered henna is mixed with an acidic liquid, like lemon juice, until it becomes a thick paste. After a few hours, it reacts with the skin leaving a reddish brown pattern that can last for two or three weeks before fading away. Along with face painting, and tattoos in general, they've become increasingly popular

"I've had henna tattoos," Pallas said.

But to make the reddish-brown dye darker, so that it stands out more against the skin, those giving the tattoos can add ingredients like PPD, a chemical dye that's commonly used as part of the mix in hair dyes. It's not supposed to be applied directly to the skin. When it is, some people can develop a severe allergic reaction to it.

Sandy Pallas said the blister in Dean's arm scabbed over. Once the scab fell off, she said, he's been left with what looks like the aftermath of a burn in the pattern of the tattoo.

"It looks like someone branded him with the New York Giants logo," she said.

Golenbock said people can treat allergic reactions with a corticosteroid cream to reduce the inflammation. Over time, he said, the skin should heal.

"They'll end up with normal skin," he said.

Notaro said he advises people against getting henna tattoos in general, simply because you can't be sure what's in the mix. Anyone who gets a bad skin reaction to such a tattoo should probably avoid hair dyes as well, he said.

But Golenbock said boardwalk vendors know their customers are probably out-of-towners anyway.

"They're there for a week, then they're gone and so is the problem," Golenbock said. "They're not going back to file a complaint or bring a lawsuit."